1. Field of the Invention
The present invention relates to an optical system including a transmissive optical element and a holographic optical element, and also relates to a method for fabricating a holographic optical element.
2. Description of the Prior Art
First, a brief description will be given of a hologram. A hologram is a photosensitive material on which the wavefront of light from an object is recorded. When the hologram is illuminated, the recorded wavefront of light is reproduced. This photographic technique is called holography.
To record a wavefront on a holographic material, interference of light is exploited. Light from an object is made to interfere with reference light that is given arbitrarily, and the resulting interference fringes are recorded. Accordingly, the two beams of light (object light and reference light) need to be able to interfere with each other, and therefore recording is achieved by the use of coherent light, typically laser light.
A reflective hologram exhibits far higher wavelength selectivity than a transmissive hologram, meaning that the former far more strikingly tends to respond to certain wavelengths but not to other wavelengths than the latter. FIG. 19 shows the diffraction wavelength width of a reflective and a transmissive hologram.
There are various types of holograms, among which phase-type holograms, which are of the volume type and which absorb little light, are suitable to obtain high diffraction efficiency and satisfactory incident light use efficiency.
A hologram is capable of reproducing a wavefront, and this property can be used to make it function as a lens. A hologram that is given such an optical function is called a holographic optical element (HOE).
Basically, a hologram reproduces a wavefront with the highest diffraction efficiency when it is illuminated with a light beam having the same wavelength and angle as the light beam with which it was fabricated. Accordingly, it is preferable to use as the incident light beam one having a wavelength peak that fits the diffraction efficiency peak of the hologram. FIG. 20 shows the relationship between the wavelength of the incident light beam and the obtained diffraction efficiency in a reflective hologram fabricated by the use of laser light having a wavelength of 532 nm. Suitable for use as the light source of the incident light beam with this hologram is a green light-emitting diode (LED) or the like having an emission peak around 530 nm. An LED has an emission wavelength band of which the half-width is 20 to 40 nm, and thus, by using it as the light source, it is possible to realize a construction that offers satisfactory energy efficiency. Needless to say, it is also possible to use as the light source a laser having the same emission wavelength as the one with which the hologram was fabricated.
A hologram can be configured as a color hologram that exhibits diffraction efficiency in a plurality of wavelength bands. This is achieved by recording the interference fringes of light of a plurality of wavelengths on a single holographic material by multiple exposure, or by laying a plurality of holograms, each having the interference fringes of light of a different wavelength recorded thereon, on one another. FIG. 21 shows an example of the relationship between the wavelength of the incident light beam and the obtained diffraction efficiency in a reflective color hologram.
HOEs are used in various optical systems in combination with other optical elements. In particular, color HOEs formed with color holograms can be used in the optical system of display apparatuses for displaying images. In such application, since a HOE is a diffractive element, it has a high dispersion, and this causes different types of incident light beam to behave in greatly varying manners. When the incident light beam is not light of a single wavelength but light spreading over a wavelength band, differences in wavelength cause chromatic aberration, resulting in lower resolution.
Some proposals have been made to solve this problem. For example, Japanese Patent Application Laid-Open No. H1-92718 proposes disposing within an optical system an optical element that corrects the chromatic aberration resulting from the dispersion by a HOE. Japanese Patent Application Laid-Open No. H6-202035 proposes disposing a plurality of HOEs within an optical system so that the chromatic aberration produced by one HOE is corrected by the other HOEs.
Moreover, when a color HOE is fabricated, it is irradiated with light of different wavelengths in such a way that the light of different wavelengths has as much the same wavefront as possible. This ensures that the reproduced wavefront is the same irrespective of wavelength. For example, Japanese Patent Application Laid-Open No. H7-210066 discloses using a curve-surfaced mirror in the optical system used to fabricate a HOE so that the HOE thus fabricated has an optical power but nevertheless permits light of different wavelengths to have the same wavefront. This exploits the fact that a reflective surface, as opposed to the refraction by a lens, does not exhibit wavelength dependency.
Using an extra optical element to correct the chromatic aberration produced by a HOE, as proposed in Japanese Patent Applications Laid-Open Nos. H1-92718 and H6-202035 mentioned above, is effective in producing high-resolution images. However, using an extra optical element in addition to a HOE leads to complicating and enlarging the optical system. A HOE is an optical element that has the advantages of being thin, light-weight, and simple. However, these advantages are spoilt if the use of an extra optical element is essential to correct the chromatic aberration produced by the HOE.
When an optical system includes, in addition to a HOE, another optical element, and this optical element transmits light, this transmissive optical element produces chromatic aberration. To correct this chromatic aberration, it is necessary to add an extra optical element for that purpose, or to use a high-performance optical element that simultaneously corrects the chromatic aberration produced by the HOE and by the transmissive optical element. This leads to further complicating and enlarging the optical system.